The present invention relates to organic electroluminescent (EL) devices. More specifically, this invention relates to a multicolor, top-side white-emitting EL device with a color filter array with improved efficiency.
Organic EL displays are typically coupled with active matrix (AM) circuitry in order to produce high performance displays. Such a display is disclosed in U.S. Pat. No. 5,550,066. However, in this type of display where light is emitted downward through the substrate, the overall area that can emit light is limited by the presence of thin film transistors (TFT""s) and other circuitry, which are opaque. The area of the display pixels that emit light relative to the total area of the pixels is known as the aperture ratio (AR) and is typically less than 50%. In order to compensate for lower AR, the device must be driven at a higher current density compared to a device with a high AR. This results in the lower AR devices having a shorter useable life than a device with a higher AR.
Therefore, much work has been done to produce AM devices which are top or surface emitting, that is where light is removed through the upper surface away from the substrate and TFT circuitry. Such a device is described in EP 1 102 317. This allows for improved AR and therefore improved performance of the display.
With a top-emitting AM device, AR could theoretically approach 100%, but is still limited by the ability to pattern all the necessary layers. That is, tolerance must be allowed between neighboring pixels for the maximum alignment error and minimum patterning resolution for each layer. This between-pixel region does not emit light and therefore lessens the AR. Many of these layers are typically patterned using photolithography techniques, which have good alignment and resolution. In the above examples of organic EL devices, in order to produce multicolor devices, such as red-green-blue (RGB) displays, the organic EL materials must be patterned as well. However, the organic materials used in organic EL films are typically incompatible with photolithography methods and therefore require other deposition techniques. For small molecule organic EL materials, the most common patterning method is deposition through a precision aligned shadow mask. Shadow mask patterning, however, has relatively poor alignment and resolution compared to photolithography. Shadow mask patterning alignment becomes even more difficult when scaled up to larger substrate sizes. Therefore, using shadow masking, the full benefits of AR gain potentially obtainable using top-emitting AM device techniques cannot be realized.
Furthermore, shadow mask patterning, which typically requires contact of the mask to the substrate, can cause defects such as scratching which reduces yield. Alignment of the shadow mask to the substrate also requires time, which reduces throughput and increases manufacturing equipment complexity.
A white-emitting EL layer can be used to form a multicolor device. Each pixel is coupled with a color filter element as part of a color filter array (CFA) to achieve a pixilated multicolor display. The organic EL layer is common to all pixels and the final color as perceived by the viewer is dictated by that pixel""s corresponding color filter element. Therefore, a multicolor or RGB device can be produced without requiring any patterning of the organic EL layers. Therefore, white CFA top-emitting AM displays offer superior AR, yield, and throughput compared to multicolor patterned top-emitting AM displays. An example of a white CFA top-emitting device is shown in U.S. Pat. No. 6,392,340.
In Japanese Patent Application Laid-Open No Heisei 4(1992)-328295, a device with improved efficiency is shown. Here, improved efficiency was achieved by optimizing the electron transport EL layer such that light generated in the direction of the viewer and light generated in the direction away from the viewer and reflected back toward the viewer by the reflective electrode interfere constructively with each other.
However, this method of optimizing the organic EL structure is incompatible with a multicolor white-emitting organic EL device. In multicolor EL devices, each of the groups of pixels that are seen by the viewer as having a different color (hereafter referred to as a pixel color group) must be optimized differently depending on the desired color or wavelength of the pixel color group. Since the white-emitting devices arc only advantageous if no patterning of the organic EL media layers is done, having one or more of the organic EL media layers with varying thickness for each pixel color group is not practical. Therefore, for white CFA top-emitting AM organic EL displays, a new structure is required to realize the benefits of optical interference optimization.
It is therefore an object of the present invention to provide an organic EL structure for use with multicolor top emission active matrix organic EL devices, which can be optimized for constructive optical interference and does not require precision patterning of the organic EL media layers for each pixel.
This object is achieved by a multicolor organic light-emitting display having an array of pixels having at least two different colors comprising:
a) a substrate;
b) a reflective layer disposed over the substrate;
c) a first transparent electrode disposed over the reflective layer;
d) a second transparent electrode spaced from the first transparent electrode;
e) organic EL media disposed between the first and second transparent electrodes and arranged to produce white light;
f) at least first and second filters of different colors disposed respectively over different predetermined pixels of the array; and
g) wherein the thickness of the first transparent electrode is separately adjusted for each different color to cause a substantial amount of the reflected component of colored light corresponding to its associated color filter to constructively interfere with a substantial amount of the non-reflected component of colored light corresponding to its associated color filter.
This invention has the advantage over prior white top-emitting organic EL devices of improved color efficiency without the need to pattern any of the organic EL Layers between the pixels. It is a further advantage that any layers that require patterning can be patterned using highly-precise photolithography techniques. The invention has the further advantage of allowing for wider selection of transparent electrode materials.